Misrepresentation of contract terms

On the 01/01/16 my cell was ****** and we went to the local MTN store to get a replacement SIM for a Blackberry that I have. While at the store Adam checked and told my husband and I that I was due for an upgrade. We explained to him that my current contract was around R319 and we would only be interested in a contract of similar value. He told us of a special running for a Samsung S6 Edge on the MTN Anytime 200 Topup contract for R379pm. We agreed that this was a good deal and took out a new 24mth contract. When the contract was printed we queried an amount of R589 on page 5 of the contract and he assured us that this was the normal rate and our new rate would be the R379 indicated on the same page (R379 is also the only amount on the Contract Summary page). At the end of Jan, the invoice was for just over R500 and we assumed that this was the R379 + the upgrade costs. End of Feb invoice was still over R500 and we queried this with MTN business and were directed back to the store. They said they would sort it out (Ref ***). End of March ditto, but this time when going to the store they pointed to the R589 amount & said this was actually our contracted rate!! We were **** to!